Method of forming an electrophoretic display having a color filter array

ABSTRACT

The invention provides an electrophoretic medium comprising at least two types of particles having substantially the same electrophoretic mobility but differing colors. The invention also provides article of manufacture comprising a layer of a solid electro-optic medium, a first adhesive layer on one surface of the electro-optic medium, a release sheet covering the first adhesive layer, and a second adhesive layer on an opposed second surface of the electro-optic medium.

REFERENCE TO RELATED APPLICATIONS

This application is a division of copending application Ser. No.15/949,996, filed Apr. 10, 2018 (Publication No. 2018/0231865), which isitself a divisional of application Ser. No. 15/228,625, filed Aug. 4,2016 (Publication No. 2016/0342065, now abandoned), which itself is adivision of application Ser. No. 14/458,513, filed Aug. 13, 2014, nowU.S. Pat. No. 10,444,590, which itself is a division of application Ser.No. 12/472,486, filed May 27, 2009 (Publication No. 2009/0225398, nowabandoned), which itself is a division of application Ser. No.10/605,024, filed Sep. 2, 2003, now U.S. Pat. No. 7,561,324, whichclaims priority from Provisional Application Ser. No. 60/319,516, filedSep. 3, 2002.

This application is related to copending application Ser. No. 10/143,046filed May 10, 2002 (Publication No. 2003/0011560, now U.S. Pat. No.7,256,766), which is a continuation-in-part of application Ser. No.09/140,862, filed Aug. 27, 1998 (now U.S. Pat. No. 7,167,155). Thisapplication is also related to copending application Ser. No. 10/145,861(Publication No. 2002/0180688, now U.S. Pat. No. 6,864,875), which is acontinuation of application Ser. No. 09/436,303, filed Nov. 8, 1999 (nowabandoned), which is turn is a divisional of copending application Ser.No. 09/289,507, filed Apr. 9, 1999 (now U.S. Pat. No. 7,075,502), whichclaims priority from Provisional Application Ser. No. 60/081,362, filedApr. 10, 1998. This application is also related to copending applicationSer. No. 10/249,957, filed May 22, 2003 (now U.S. Pat. No. 6,982,178),which claims priority from Application Ser. No. 60/319,300, filed Jun.10, 2002 and Application Ser. No. 60/320,186, filed May 12, 2003. Theentire contents of all the aforementioned applications, and of allpatents and applications referred to below, are herein incorporated byreference.

BACKGROUND OF INVENTION

This invention relates to electro-optic displays. More specifically,this invention relates to components for use in electro-optic displays,novel color filters for use in such displays and methods for preparingsuch filters. This invention also relates to novel methods forcontrolling the color of electrophoretic displays.

Electro-optic displays comprise a layer of electro-optic material, aterm which is used herein in its conventional meaning in the art torefer to a material having first and second display states differing inat least one optical property, the material being changed from its firstto its second display state by application of an electric field to thematerial. The optical property is typically color perceptible to thehuman eye, but may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

Most aspects of the present invention are intended for use withelectro-optic displays containing an electro-optic medium which is asolid (such displays may hereinafter for convenience be referred to as“solid electro-optic displays”), in the sense that the electro-opticmedium has solid external surfaces, although the medium may, and oftendoes, have internal liquid- or gas-filled spaces, and to methods forassembling displays using such an electro-optic medium. Thus, the term“solid electro-optic displays” includes encapsulated electrophoreticdisplays, rotating bichromal member displays, electrochromic displays,microcell displays, and other types of displays as discussed below.

One type of electro-optic display is a rotating bichromal member type asdescribed, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782;5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and6,147,791 (although this type of display is often referred to as a“rotating bichromal ball” display, the term “rotating bichromal member”is preferred as more accurate since in some of the patents mentionedabove the rotating members are not spherical). Such a display uses alarge number of small bodies (typically spherical or cylindrical) whichhave two or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedto applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface.

Another type of electro-optic medium is an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. No. 6,301,038, International Application Publication No. WO01/27690, and in copending application Ser. No. 10/249,128, filed Mar.18, 2003.

Another type of electro-optic display, which has been the subject ofintense research and development for a number of years, is theparticle-based electrophoretic display, in which a plurality of chargedparticles move through a suspending fluid under the influence of anelectric field. Electrophoretic displays can have attributes of goodbrightness and contrast, wide viewing angles, state bistability, and lowpower consumption when compared with liquid crystal displays.Nevertheless, problems with the long-term image quality of thesedisplays have prevented their widespread usage. For example, particlesthat make up electrophoretic displays tend to settle, resulting ininadequate service-life for these displays.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a liquid suspension medium, and a capsule wallsurrounding the internal phase. Typically, the capsules are themselvesheld within a polymeric binder to form a coherent layer positionedbetween two electrodes. Encapsulated media of this type are described,for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; and 6,580,545;and U.S. Patent Applications Publication Nos. 2002/0019081;2002/0021270; 2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677;2002/0090980; 2002/0106847; 2002/0113770; 2002/0130832; 2002/0131147;2002/0145792; 2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378;2003/0011560; 2003/0011867; 2003/0011868; 2003/0020844; 2003/0025855;2003/0034949; 2003/0038755; 2003/0053189; 2003/0076573; 2003/0096113;2003/0102858; 2003/0132908; 2003/0137521; and 2003/0137717; andInternational Applications Publication Nos. WO 99/67678; WO 00/05704; WO00/38000; WO 00/38001; WO 00/36560; WO 00/67110; WO 00/67327; WO01/07961; and WO 01/08241.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display in whichthe electrophoretic medium comprises a plurality of discrete droplets ofan electrophoretic fluid and a continuous phase of a polymeric material,and that the discrete droplets of electrophoretic fluid within such apolymer-dispersed electrophoretic display may be regarded as capsules ormicrocapsules even though no discrete capsule membrane is associatedwith each individual droplet; see for example, the aforementioned2002/0131147. Accordingly, for purposes of the present application, suchpolymer-dispersed electrophoretic media are regarded as sub-species ofencapsulated electrophoretic media.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes; andother similar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the suspending fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, International Applications Publication No. WO 02/01281, andpublished US Application No. 2002-0075556, both assigned to SipixImaging, Inc.

Most types of electro-optic media have only a limited number of opticalstates, for example a dark (black) state, a light (white) state and, insome cases, one or more intermediate gray states. Accordingly, toconstruct a full color display using such media, it is common practiceto place an electro-optic medium adjacent a color filter having, forexample, multiple red, green and blue areas, and to provide a drivingarrangement for the electro-optic medium which permits independentcontrol of the medium adjacent each red, green or blue area. Certainapplications of color filters with electrophoretic displays aredescribed in the aforementioned application Ser. No. 09/289,507. Theaforementioned 2003/0011560 describes ways for modifying the opticalproperties of electrophoretic displays by incorporating an opticalbiasing element in any one of several components of the display.

The present invention seeks to provide improvements in color filtersused in electro-optic displays, and in ways of generating color in suchdisplays.

SUMMARY OF INVENTION

In one aspect, the present invention provides an electrophoretic mediumcomprising a plurality of electrically charged particles dispersed in asuspending fluid, the particles comprising at least two types ofparticles having substantially the same electrophoretic mobility butdiffering colors. This aspect of the invention may hereinafter bereferred to as the “custom color” electrophoretic medium.

In this custom color electrophoretic medium, the electrically chargedparticles are desirably formed from an inorganic pigment, which may becoated with a coating selected from silica and silica alumina. Theelectrically charged particles may be coated with a polymer. Theelectrically charged particles and the suspending fluid may be heldwithin at least one capsule.

This invention also provides a process for forming a custom colorelectrophoretic medium of the invention. This process comprises:

mixing at least two pigments having differing colors to form a mixedpigment;

subjecting the mixed pigment to at least one surface treatment; and

dispersing the surface-treated mixed pigment in a suspending fluid toform at least two types of particles having substantially the sameelectrophoretic mobility but differing colors.

In this process, the surface treatment may comprise treating the mixedpigment with a silane coupling agent to provide sites at which a polymercan be attached to the mixed pigment, and thereafter treating thesilylated mixed pigment with at least one monomer and oligomer underconditions effective to cause polymer for form of the mixed pigmentsurface.

In another aspect, the present invention provides an electro-opticdisplay element comprising:

an electro-optic display medium;

an optical biasing element arranged to modify an optical characteristicof the electro-optic display element; and

an addressing electrode to address the electro-optic display medium,

wherein the color of the optical biasing element varies in differentportions of the electro-optic display element, so that the opticalbiasing element forms a color filter.

This aspect of the invention may hereinafter be referred to as the“internal color filter” display element.

In such an internal color filter display element, the electro-opticdisplay medium may be an electrophoretic medium comprising a suspendingfluid, a plurality of electrically charged particles suspended in thesuspending fluid and capable of moving therethrough on application of anelectric field to the suspending fluid, and at least one capsule havinga capsule wall surrounding the suspending fluid and the electricallycharged particles, the display element optionally comprising a bindersurrounding the capsules and/or a lamination adhesive disposed adjacentthe electrophoretic medium and/or a front electrode disposed between theelectrophoretic medium and a viewing surface of the display, and theoptical biasing element may be disposed in at least one of the capsulewall, the binder, the lamination adhesive and the front electrode.

Another, related aspect of the invention provides an electro-opticdisplay comprising a layer of a solid electro-optic medium, at least oneelectrode arranged adjacent the layer of electro-optic medium so as toapply an electric field thereto, and a color filter array disposedbetween the electro-optic medium and the electrode, the resistance ofthe color filter array being not substantially greater than that of thelayer of electro-optic medium.

In such an electro-optic display, the color filter array will typicallyhave a volume resistivity not greater than about 10¹⁰ ohm cm.

In another aspect, this invention provides an electrophoretic mediumcomprising a plurality of capsules, each capsule comprising a suspendingfluid, a plurality of electrically charged particles suspended in thesuspending fluid and capable of moving therethrough on application of anelectric field to the suspending fluid, and a capsule wall surroundingthe suspending fluid and the electrically charged particles, the mediumfurther comprising a color filter array having a plurality ofnon-rectangular pixels. In preferred forms of this electrophoreticdisplay, the pixels of the color filter array are hexagonal, square, ortriangular, preferably equilateral triangular.

In another aspect, this invention provides an article of manufacturecomprising:

a layer of a solid electro-optic medium having first and second surfaceson opposed sides thereof;

a first adhesive layer on the first surface of the layer of solidelectro-optic medium;

a (first) release sheet disposed on the opposed side of the firstadhesive layer from the layer of solid electro-optic medium; and

a second adhesive layer on the second surface of the layer of solidelectro-optic medium.

A preferred form of this article of manufacture further comprises asecond release sheet disposed on the opposed side of the second adhesivelayer from the layer of solid electro-optic medium.

In this article of manufacture, the electro-optic medium may be anelectrophoretic medium comprising a plurality of capsules, each capsulecomprising a suspending fluid, a plurality of electrically chargedparticles suspended in the suspending fluid and capable of movingtherethrough on application of an electric field to the suspendingfluid, and a capsule wall surrounding the suspending fluid and theelectrically charged particles. Also, the first and second adhesivelayers may extend beyond the periphery of the layer of electro-opticmedium; as described in more detail below, this provides a convenientway for form an edge seal for the display.

In another aspect, this invention provides an article of manufacturecomprising:

a layer of a solid electro-optic medium having first and second surfaceson opposed sides thereof;

a first release sheet covering the first surface of the layer of solidelectro-optic medium; and

a second release sheet covering the second surface of the layer of solidelectro-optic medium.

The foregoing articles of manufacture may hereinafter be referred to asthe “double release film” of the invention.

This invention also provides a process for forming an electro-opticdisplay using the double release film of the invention. This processcomprises:

providing an article of manufacture comprising a layer of a solidelectro-optic medium having first and second surfaces on opposed sidesthereof, a first adhesive layer on the first surface of the layer ofsolid electro-optic medium, a release sheet disposed on the opposed sideof the first adhesive layer from the layer of solid electro-opticmedium; and a second adhesive layer on the second surface of the layerof solid electro-optic medium;

laminating the article to a front substrate via the second adhesivelayer, thereby forming a front subassembly;

removing the release sheet from the front subassembly; and

laminating the front subassembly via the first adhesive layer to abackplane comprising at least one electrode, thereby forming theelectro-optic display.

In this process, the front substrate may comprise an electrode and/or acolor filter array. The article of manufacture may comprise a secondrelease sheet covering the second adhesive layer, and the processcomprise removing the second release sheet from the second adhesivelayer prior to laminating the article to the front substrate.

In another aspect, this invention provides a process for forming a colorfilter array, the process comprising:

imaging a photosensitive film to form a color filter array patternthereon; and

thereafter depositing a conductive layer on to the photosensitive film.

In another aspect, this invention provides a process for forming anelectrophoretic display, the process comprising:

providing a color filter array;

providing an electrophoretic medium comprising a plurality of capsules,each capsule comprising a suspending fluid, a plurality of electricallycharged particles suspended in the suspending fluid and capable ofmoving therethrough on application of an electric field to thesuspending fluid, and a capsule wall surrounding the suspending fluidand the electrically charged particles;

depositing the electrophoretic medium on to a color filter array to forma coated color filter array; and

thereafter laminating the coated color filter array to a backplanecomprising at least one pixel electrode.

In this process, the surface of the color filter array may be surfacetreated prior to the deposition to produce regions of varying surfaceenergy on the surface.

In another aspect, this invention provides a process for depositing anelectrophoretic medium on an electrode, the process comprising:

providing an electrode;

providing an electrophoretic medium comprising a plurality of capsules,each capsule comprising a suspending fluid, a plurality of electricallycharged particles suspended in the suspending fluid and capable ofmoving therethrough on application of an electric field to thesuspending fluid, and a capsule wall surrounding the suspending fluidand the electrically charged particles;

surface treating the electrode to produce regions of varying surfaceenergy thereon; and

depositing the electrophoretic medium on the surface-treated electrode.

Finally, this invention provides a process for forming anelectrophoretic display, the process comprising:

providing a front substrate;

providing a backplane;

providing an electrophoretic medium comprising a plurality of capsules,each capsule comprising a suspending fluid, a plurality of electricallycharged particles suspended in the suspending fluid and capable ofmoving therethrough on application of an electric field to thesuspending fluid, and a capsule wall surrounding the suspending fluidand the electrically charged particles;

surface treating a surface of the front substrate to promote wettingthereof by the capsules;

surface treating a surface of the backplane to promote dewetting thereofby the capsules;

assembling the front substrate and the backplane together with thetreated surfaces thereof facing each other, and with a gap between thetreated surfaces; and

introducing the electrophoretic medium into the gap, whereby thecapsules of the electrophoretic medium pack against the treated surfaceof the front substrate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 of the accompanying drawings is a schematic cross-section throughpart of an encapsulated electrophoretic display in which a color filterarray is provided in the capsule walls;

FIGS. 2A, 2B and 2C show three different arrangements of pixels in acolor filter array of the present invention;

FIG. 3 is a schematic cross-section through a double release film of thepresent invention; and

FIG. 4 is a schematic cross-section, similar to that of FIG. 3, througha second release film of the present invention provided with an edgeseal.

DETAILED DESCRIPTION

The present invention provides several different improvements in colorfilters and other aspects of electro-optic displays, and in ways ofgenerating color in such displays. These various improvements can beused alone or in various combinations (for example, a single displaymight use a color filter array having non-rectangular pixels produced bythe imaging process of the invention. For convenience, the variousaspects of the present invention will hereinafter be describedseparately, but it must always be remembered that multiple aspects ofthe invention may be used in a single electro-optic display or componentthereof.

Part A—Custom Colors in Electrophoretic Displays

As discussed in some of the aforementioned E Ink and MIT patents andpublished applications, in an electrophoretic display one alternative tothe use of color filters is to use multiple types of capsules capable ofdisplaying differing colors. For example, the aforementioned2002/0180688 shows, in FIG. 31, one pixel of an encapsulatedelectrophoretic display, this pixel comprising three sub-pixels, each ofwhich comprises a single capsule capable of displaying three colors.Although these are not the colors actually described with reference tothis FIG. 31, it will readily be understood by those skilled in thetechnology of electro-optic displays that a full color RGB display couldbe produced using capsules capable of white/black/red, white/black/greenand white/black/blue optical states.

One major commercial application of electro-optic displays is inadvertising, and in advertising materials it is desirable to be able tocustomize the colors displayed for particular customers. For example,many major corporations have established corporate practices whichrequire that certain color logos and/or trademarks be presented in anabsolutely consistent manner, with the proper colors of each portion ofthe relevant logo or trademark being specified in terms of the Pantone(“PANTONE” is a Registered Trademark) or a similar color system. Thus,such corporations may require that (say) the blue state of an RGBelectrophoretic display be customized to render a blue portion of theircorporate logo accurately, even at the cost of some reduction in thecolor gamut of the display (the range of colors capable of beingdisplayed by the display).

At first glance, such customization of colors in an electrophoreticdisplay appears to pose formidable obstacles. Determining the type ofparticles to use in an electrophoretic display is a complex processwhich needs to take account not only of the color of the particle butalso, inter alia, its ability to maintain a consistent electrical charge(and thus a consistent electrophoretic mobility), its tendency toaggregate, its color fastness against light and its compatibility withother components of the electrophoretic medium, for example thesuspending fluid in which the electrophoretic particles are suspendedand the capsule wall. Carrying out all the necessary tests is atime-consuming and expensive process, which cannot economically becarried out every time a customer requires a custom color in a display.Furthermore, in practice it is found that organic pigments tend not tobe useful in electrophoretic displays because they degrade too rapidly,so that commercial displays use metal oxide or similar inorganicpigments, and it is far less easily to vary the exact color of suchinorganic pigments than it is organic pigments.

However, it has now been realized that, in an electrophoretic display,all of the electrophoretic particles of a particular type used toproduce a particular color (for example, the blue particles shown in theaforementioned FIG. 31) need not be of the same color, provided that allof the same type of particles have similar electrophoretic mobilities sothat they do not segregate during operation of the display. Theindividual electrophoretic particles, which typically have diameters ofthe order of 1 μm, are far too small to be visible to an observer of thedisplay. Accordingly, provided that the particles do not segregateduring operation of the display, an observer sees only the average colorof all the particles of a particular type. For example, it is well knownto those skilled in color mixing that the apparent degree of saturationof many blues can be improved by adding a small proportion of magenta tothe blue. Accordingly, if for example a particular customer required theblue state of the aforementioned white/black/blue sub-pixels to have adegree of saturation greater than could be achieved by the availableblue particles, the sub-pixel could contain a mixture of a majorproportion of blue particles and a minor proportion (say about 10 percent) of magenta particles.

This ability to use mixtures of differently colored particles as asingle type of electrophoretic particles greatly simplifies the problemof customizing colors in an electrophoretic display. If one prepares aset of three or four electrophoretic particles having differing colors(say RGB, RGBK, CMY or CMYK) but substantially the same electrophoreticmobility, pixels having any desired color within the gamut availablefrom the set can be prepared by simply mixing and encapsulatingappropriate amounts of the various particles to form the desired color.

Thus, as already mentioned in one aspect the present invention providesan electrophoretic medium comprising a plurality of electrically chargedparticles dispersed in a suspending fluid, the particles comprising atleast two types of particles having substantially the sameelectrophoretic mobility but differing colors.

Although this aspect of the present invention is described hereinprimarily with regard to full color displays, it is of course notconfined to such displays. For example, the present invention may beuseful in simple flashing displays in which a layer of anelectrophoretic medium is confined between two electrodes, which applydrive pulses of alternating polarities to the electrodes, and in whichthe layer of electrophoretic medium comprises different types ofcapsules deposited in different areas, each type of capsule having (say)a black optical state and a second optical state of another color. Sucha display may be used to flash a corporate logo or other symbol.

Technology is available which substantially reduces the difficulties ofensuring that electrophoretic particles formed from different pigmentshave substantially the same electrophoretic mobility. The aforementioned2002/0185378 and the corresponding WO 02/093246 describe a process forforming electrophoretic particles in which a pigment is first coatedwith a layer of silica (other oxides may also be used) and then apolymer is formed chemically bonded to the silica; the polymer maycontain electrically charged groups to enhance the electrophoreticmobility of the particle. In such silica/polymer coated particles, thesurface charge which is responsible for the electrophoretic mobility ofthe particle is essentially confined to the silica layer and/or thepolymer, so that the nature of the underlying pigment is essentiallyirrelevant to the electrophoretic mobility of the particle. Accordingly,provided that the particle size of the pigment is controlled, a varietyof pigments can be coated in this manner to produce electrophoreticparticles having substantially the same electrophoretic mobility.

Furthermore, if the scale of production needed warrants it, mixing ofthe different pigments required to produce a display with a custom colorcan be effected at an early stage in the multi-stage process required toproduce the display, with the resultant mixture of pigments thereaftercarried as a single component through the subsequent stages of theprocess. By way of example, consider the preferred process for producingelectrophoretic particles described in aforementioned 2002/0185378 andthe corresponding WO 02/093246. This process comprises (a) forming asilica or silica/alumina shell around a raw pigment; (b) treating thesilica-coated pigment with a silane coupling agent to provide sites atwhich a polymer can be attached to the pigment; and (c) treating thesilylated pigment would at least one monomer or oligomer underconditions effective to cause polymer to form on the pigment surface(certain preferred forms of the process require multiple polymerizationsteps and/or further processing to modify the chemical characteristicsof the polymer originally formed). In such a process, the blending oftwo or more pigments to provide a custom color in the final displaycould be effected at various stages, as follows:

1. Blending of Raw Pigments:

Raw untreated pigments may be blended to obtain the desired color beforethe first modification of the surface. The blended pigment mixture isthen coated with silica or silica/alumina followed by the silanetreatment and polymer formation steps.

At first glance, this might appear to be the most preferred method toassure that the blended pigments have been treated in the same mannerunder identical conditions thus leading to blended pigments withidentical surface chemistries. On the other hand, it may be difficult tojudge the color in the final display from that obtained by raw pigmentblending because of color alterations due to the inorganic coating,pigment wetting when in the suspending fluid (silica is used to coatpigments in paint applications to prevent this wetting effect, butwetting must still be considered) and alterations in the pigment colorwhen placed behind the capsule wall, conductive, binder, adhesive,filter layer and other layers conventionally used in electrophoreticdisplays, as described in the aforementioned E Ink and MIT patents andapplications. These color alterations may require repeated iterations ofthe blending process and the production of displays from the blendedpigment, with considerable increase in expense and development time forcustom color displays.

2. Blending of Pigments After Formation of Inorganic Shell

In this route, raw pigments are first modified independently with asilica or silica/alumina shell then blended to achieve the desiredcolor, and thereafter the blended pigment is subjected to silanetreatment and polymer formation.

3. Blending of Pigments After Formation of Inorganic Shell and SilaneTreatment

In this route, raw pigments are first modified independently with asilica or silica/alumina shell and silane treatment. The pigments arethen blended to obtain the desired color and the blended pigmentssubjected to polymer formation.

4. Blending of Pigments Formation of Inorganic Shell, Silane Treatmentand Polymer Formation

In this route, raw pigments are first modified independently with asilica or silica/alumina shell, followed by silane treatment and polymerformation. The pigments are then blended to achieve the desired color.

This route ensures that the surface modification of the pigments doesnot alter the color of the pigments because the pigments are blendedafter all chemical modifications have been completed. Nevertheless thecolor of the blended pigment may still change after incorporation intoan internal phase and placement behind a capsule wall, binder, adhesive,and other layers as discussed above. However, this route does may giverise to problems in ensuring that the pigments in the final blend havebeen treated in the same manner under identical conditions thus leadingto identical surface chemistries such as the thickness/coverage of thesilica or silica/alumina layer, coverage/quality/density of the silanedeposition and thickness/molecular weight/density of the polymer.

Part B—“Internal” Color Filters

Conventional color filter arrays (CFA's) for use in electro-opticdisplays comprise a layer of material colored in an appropriate mannerand disposed adjacent the viewing surface of the display, “outside” thefront electrode, i.e., between the front electrode and the viewer. Sucha conventional CFA is necessarily separated from the electro-optic layerby at least the thickness of the front electrode and possibly otherlayers, for example a substrate on which the front electrode is mountedand/or a binder in which electrophoretic capsules are held and/or thepolymeric matrix in a rotating bichromal member display. This separationbetween the CFA and the electro-optic layer gives rise to parallaxproblems, and these problems are exacerbated by the wide viewing anglesof many electro-optic displays. Such parallax problems adversely affectthe quality of an image on the display, especially when the display isviewed at a large angle to the perpendicular to the viewing surfacethereof.

Another problem with conventional CFA's is aligning the CFA with thepixel electrodes used to drive the display; it will be appreciated thatany misalignment between the CFA and the pixel electrodes will causecross-talk between the various color channels of the image on thedisplay. Alignment of CFA's with pixel electrodes is more difficult inmany types of electro-optic displays than in liquid crystal displays,since in liquid crystal displays the CFA and the pixel electrodes can bealigned optically before the liquid crystal itself is introduced intothe display, whereas in, for example, electrophoretic or rotatingbichromal member displays, this approach is not possible since thenormal method for manufacturing such displays is to deposit theelectro-optic layer on to the CFA, and then to laminate the combinedelectro-optic layer/CFA to a backplane containing the pixel electrodes.Since an electrophoretic or rotating bichromal member electro-opticlayer is essentially opaque, the pixel electrodes are not visiblethrough the electro-optic layer during the lamination procedure, thusprecluding optical alignment of the CFA with the pixel electrodes.

As already mentioned, the aforementioned 2003/0011560 describes ways formodifying the optical properties of electrophoretic displays byincorporating an optical biasing element in any one of severalcomponents of the display. It has now been realized that this opticalbiasing element can serve as a color filter for the display, providedthat the optical biasing element has varying colors in different areasof the display to provide the necessary color filter elements.

The possible locations of such an optical biasing element/color filterwill vary with the type of electro-optic display. For example, in anencapsulated electrophoretic display the color filter may be provided(a) in the walls of the capsules; (b) in the binder which surrounds thecapsules; (c) in a lamination adhesive used to secure theelectrophoretic medium to a substrate which forms the viewing surface ofthe display; or (d) in the front electrode of the display. (Note thatsince an electrophoretic medium is largely opaque, although theaforementioned 2003/0011560 describes optical biasing elements which lie“behind” the electrophoretic medium, i.e., on the opposed side of themedium from the viewing surface of the display, an optical biasingelement serving as a color filter needs to be located either within theelectrophoretic medium itself or between the electrophoretic medium andthe viewing surface of the display.) In a microcell electrophoreticdisplay, the color filter may be provided in the front sheet which sealsthe microcells, or in locations corresponding to (c) or (d) above. In arotating bichromal member display, the optical biasing element/colorfilter may be provided in the polymeric matrix which surrounds therotating members (this matrix essentially corresponding to the binder ofan encapsulated electrophoretic display) or in locations (c) or (d)above.

FIG. 1 of the accompanying drawings is a schematic cross-section througha preferred embodiment of the invention in which a color filter isprovided in the capsule walls of an encapsulated electrophoretic display(generally designated 100). The display 100 comprises an encapsulatedelectrophoretic medium (generally designated 102) comprising a pluralityof capsules 104, 104′, 104″, each of which contains a suspending liquid106 and dispersed therein a plurality of positively charged blackparticles 108 and a plurality of negatively charged, white particles116. (The triangular shape of the particles 108 and the circular shapesof the particles 116 are used purely by way of illustration to enablethe particles to be distinguished easily in the accompanying drawings,and in no way correspond to the physical forms of the actual particles,which are typically substantially spherical.)

The display 100 further comprises a common, transparent front electrode110, which forms a viewing surface through which an observer views thedisplay 100, and a plurality of discrete rear electrodes 112, 112′,112″, each of which defines one pixel of the display 100. For ease ofillustration and comprehension, FIG. 1 shows only a single microcapsuleforming the pixel defined by each rear electrode, although in practice alarge number (20 or more) microcapsules are normally used for eachpixel. The rear electrodes 112, 112′, 112″ are mounted upon a substrate114.

The display 100 is an RGB display with the red, green and blue pixelsarranged in cyclically repeating columns; it will be appreciated thatalthough only one electrode is shown in each column in FIG. 1, inpractice each column will contain a large number of electrodes, thepotential on each of which is independently controlled by conventionalactive matrix drive circuitry (not shown). There is no external colorfilter; instead, an “internal” color filter is provided by dyeing thewalls of the capsules 104, 104′, 104″. As indicated by the differencesin shading in FIG. 1, the walls of capsule 104 are dyed to transmit redlight, the walls of capsule 104′ are dyed to transmit green light andthe walls of capsule 104″ are dyed to transmit blue light.

FIG. 1 shows the display 100 with the rear electrodes 112 and 112″ heldnegative with respect to the common front electrode 110, but with therear electrode 112′ held positive with respect to the common frontelectrode 110. Accordingly, in capsules 104 and 104″, the whiteparticles 116 lie adjacent the viewing surface of the display, so thatthese pixels appear red and blue respectively. However, in capsule 104′,the black particles 108 lie adjacent the viewing surface of the display,so that this pixel appears black.

It will be appreciated that displays of the type illustrated in FIG. 1do not need to use a RGB color scheme but could, for example, use a CMYcolor scheme. Indeed, the latter may be preferred for a reflectiveelectro-optic medium, such as the electrophoretic medium shown in FIG.1, since a CMY color scheme typically absorbs less of the incoming lightand may thus produce brighter colors.

Providing a CFA by dyeing the walls of microcapsules in anelectrophoretic display in the manner described above with reference toFIG. 1 substantially eliminates the aforementioned parallax problem inthat there is no longer any separation between the color filter arrayand the electro-optic layer of the display. Similar advantages asregards parallax problems are achieved by providing the CFA is thebinder of an electrophoretic layer or in the polymeric matrix of arotating bichromal member layer.

A wide variety of techniques may be used to produce displays with theinternal CFA's of the present invention. These techniques may be dividedinto two main groups, “preformed” CFA's, in which the color isintroduced into a display component before the component is formed intoa display, and “in situ” CFA's, in which the CFA is formed in situ inthe final display or in some sub-assembly thereof. An example of apreformed CFA is dyeing (or possibly treating with a pigment) capsulesto form the red, green and blue transmitting capsule walls as describedabove with reference to FIG. 1, and then laying down the three types ofmicrocapsules in stripes aligned with the pixel electrodes to form thedisplay illustrated in FIG. 1. In general, however, in situ CFA's may bemore easily adaptable for mass production of displays. It will beappreciated that, for example, in the display of FIG. 1, all thecapsules are identical apart from the staining of the capsule walls, sothat the display may be produced by coating a uniform layer of capsulesand then dyeing the capsule walls, and optionally the surroundingbinder. to produce the internal CFA. Note that although such a processmay result in a single capsule extending across two adjacent electrodes,this has little or no effect on the properties of the display, since ithas been found empirically that adjacent parts of the same capsule canhave differing optical states when exposed to differing electricalfields.

In situ processes for the formation of CFA's may be divided into twomain types. In the first type, an external coloring agent (for example,a dye or pigment, or a reagent which undergoes a color-forming reactionwith a component of the display in which the CFA is to be formed) isapplied imagewise to the display. For example, capsule walls may bestained by a pigment or dye which may be deposited using coating orprinting techniques, such as registered slot coating of colored stripes,spraying through a mask, ink jet, offset, flexo-gravure or itaglioprinting. In the second type, color is developed in the component of thedisplay in which the CFA is to be formed using techniques based uponphotographic processes. For example, chemical additives may be coatedonto the capsule walls such that they resemble color film. Then, byprojecting a image of the desired CFA on to the capsule coating, thecapsules may be tinted to achieve proper coloration. In using the term“photographic processes”, we do not intend to restrict the color-formingmethods used to those based upon the photosensitivity of silver halides,and indeed the use of silver-based color formation will generally not bedesirable since there are substantial practical difficulties in removingsilver and silver halide from the CFA after the exposure. However,certain color-forming reactions are known which are strongly sensitiveto wavelength, cf. U.S. Pat. No. 4,602,263, and such color-formingreactions may be used to form the CFA's. For example, capsules could beprepared in which the walls contain mixtures of red-, green- andblue-dye forming compounds, these three compounds being sensitive tothree different wavelengths of radiation (typically different infra-redwavelengths). After incorporation of the capsules into the display, therelevant areas of the display are exposed to radiation, preferably fromlasers, of the different wavelengths to turn these areas red, green andblue, thus forming the CFA. Alternatively, the nanoparticlecolor-forming technology described in U.S. Pat. Nos. 6,323,989 and6,538,801 and in copending application Ser. No. 10/065,617 (PublicationNo. 2003/0096113) may be used to produce the necessary color changes.For example, this patent and application disclose tethered nanoparticleswhich change color when the tether is broken by, for example,ultra-violet or other radiation. By including tethered nanoparticlesinto the capsule walls and then exposing the relevant areas of thedisplay to the tether-breaking radiation, the color changes needed toform a CFA could be effected.

The in situ methods described above for formation of CFA's may becarried out on an electro-optic layer on a double-release film (seeSection E below) rather than on an assembled display.

In situ formation of the CFA may be used to reduce or eliminate the CFAalignment problem discussed above by using the display itself to providealignment marks needed for accurate alignment of the CFA with the pixelelectrodes. For example, consider the aforementioned display containingcapsules in which the walls contain mixtures of red-, green- andblue-dye forming compounds, these three compounds being sensitive tothree different wavelengths of radiation. Once such a display has beenassembled with an active matrix backplane, a portion of the backplanemay be activated to form an image on the display. For example, thebackplane might be activated such that the areas intended to be red andgreen in the final CFA are dark in color, whereas the areas intended tobe blue are white. The resultant image can be used as alignment marks toexpose the white areas of the display to the radiation needed to turnthe capsules in these areas transmissive of blue light. Next, forexample, the blue and intended green areas can be set dark, while theintended red areas are set white. It will be apparent that the entireCFA may be produced in this manner correctly aligned with the pixelelectrodes.

In addition to the internal CFA's the displays of the present inventionmy include any one or more of antireflective coatings, microlens arrays,holographic filters, and brightness enhancement films. It may also bedesirable to incorporate a barrier film onto the front of the displaystack.

Part C—Color Filters with Non-Rectangular Pixels

Traditional CFA's used in liquid crystal displays are not optimallysuited for use in encapsulated electrophoretic displays. An encapsulatedelectrophoretic medium tends to exhibit a slightly degraded opticalstate near the boundaries between capsules (i.e., at the capsule walls)compared to the optical state exhibited near the center of a capsule.

It has now been realized that these effects due to the capsule walls canbe minimized by careful choice of the shapes of the pixels in the CFA;the pixel electrodes used to control the display must of course matchthe shapes of the CFA pixels. Designs which increase the likelihood thatthe capsule walls will lie in the gaps between adjacent pixels, ascompared with the (elongate, non-square) rectangular pixels typicallyused in liquid crystal displays are preferred. The CFA pixels may havethe form of hexagons (see FIG. 2A), squares (FIG. 2B) or triangles,preferably equilateral triangles (FIG. 2C). In the accompanyingmonochrome drawings, the colors of the various pixels have beenindicated by R, G and B for red, green and blue respectively. Althoughin theory pentagonal pixels could be used, these and similar shapes, forexample octagonal pixels, are not favored because they do not close packon a surface. Hexagonal pixels are especially favored because capsulesideally close pack in a hexagonal grid; square pixels are alsofavorable.

Part D—Conductive Color Filters between Electrodes

As already indicated, conventional CFA's developed for use in liquidcrystal displays comprise a layer of material colored in an appropriatemanner and disposed adjacent the viewing surface of the display,“outside” the front electrode, i.e., between the front electrode and theviewer. Such conventional CFA's are manufactured on glass substrates andare formed from a plurality of films (usually referred to as a “stack”),typically including a black mask (usually formed of chromium), red,green and blue colored photoresist layers and a conductive layer,typically formed of indium-tin-oxide (no), which forms the frontelectrode of the display.

Also as already indicated, certain types of electro-optic display,especially encapsulated electrophoretic displays, microcell displays androtating bichromal member displays, have the advantage that they can beformed on flexible substrates, such as plastic films of poly(ethyleneterephthalate) or poly(ethylene naphthalate). While it is technicallypossible to use conventional CFA's of the type described in thepreceding paragraph with electro-optic displays formed on plastic films,such conventional CFA's are not well adapted for such use. A blackchromium mask poses difficulties on plastic substrates, since this metalis typically patterned using etchants that tend to harm plastics;accordingly, it is desirable to eliminate the black mask.

It is also desirable, the present inventors have realized, if an“internal” CFA as described in Part B hereof is not employed, tointerchange the order of the colored layers and the conductive layer.Such an interchange facilitates manufacturing, since plastic filmscoated with conductive materials such as ITO are readily availablecommercially and can be used as starting materials in the process ofmanufacturing the CFA.

However, such an interchange necessarily places the colored layersbetween the electrodes of the display and, at any given operatingvoltage between the electrodes, reduces the voltage across theelectro-optic medium itself, since the voltage across the electro-opticmedium is equal to the operating voltage minus the voltage drop acrossthe colored layers. The reduced voltage across the electro-optic mediumtypically slows the switching rate of the medium. Although it ispossible to compensate for the reduced voltage across the electro-opticmedium by increasing the operating voltage, this is generallyundesirable since it increases the energy consumption of the display andmay require modification of electrical circuitry to produce and/orhandle the increased voltage. Furthermore, the electrical resistivity ofmany commercial photoresists is so high that a very large, and oftenunacceptable, increase in operating voltage would be required tocompensate for the presence of the colored layers between theelectrodes.

Accordingly, it is desirable to increase the conductivity of thematerial used to form the colored layers so that the resistance of thecolored layers does not substantially exceed that of the electro-opticlayer, and indeed is desirably less than that of the electro-opticlayer. Given the low conductivity of most types of electro-optic media,the colored layers should have a volume resistivity not greater thanabout 10¹⁰, and preferably not greater than about 10⁹ ohm cm. Thecolored layers may be made more conductive by doping with conductivenanoparticles, for example nanoparticles of silver, gold, aluminum,platinum or carbon; although these materials are opaque in bulk form,they can be made into nanoparticles small enough (typically less than 10nm in diameter) that they do not substantially scatter light, and thusdo not interfere with the optical problems of the colored layers. Otherorganic or inorganic pigments that have favorable electrical and opticalproperties can also be used, as can conductive polymers and othermaterials, although it is of course necessary to take account of theoverall optical properties of the display after addition of thesematerials. The techniques described in the aforementioned 2003/0011867may be used to control the conductivity of the colored layers. Also,since the colored layers are disposed between the electrodes of theelectro-optic display, the electrical characteristics, and the variationof such characteristics with environmental parameters such astemperature, may affect the operating characteristics of the display inessentially the same way as any other layer, for example a laminationadhesive, disposed between the electrodes of the display. The reader isreferred to the aforementioned 2003/0025855 for a detailed summary ofthe desirable characteristics of lamination adhesives used inelectro-optic displays; most of these characteristics are directlyapplicable to colored layers used as internal color filter arrays inaccordance with the present invention.

Part E—Double Release Film

As already indicated, the methods used in liquid crystal displays forincorporation of CFA's are not well adapted for use with other types ofelectro-optic media. In particular, as already indicated, the techniquesused to assemble liquid crystal displays do not transfer well for usewith other types of electro-optic media. In the conventional process forassembly of liquid crystal displays, a front assembly is formedcomprising a substrate, color filter layers and a conductive layer, andthis front assembly is aligned with and secured to a rear assemblycomprising pixel electrodes and associated circuitry, a narrow cell gapbeing maintained between the two assemblies by the use of spacers. Thiscell gap is then evacuated and filled with the liquid crystal materialitself by dipping the combined assemblies into a bath of the liquidcrystal material.

Although in theory one could form an encapsulated electrophoreticdisplay in a similar manner, in practice this assembly technique ishighly undesirable because when injected into a narrow cell gap, thecapsules will not pack closely, as is desirable for optimum opticalperformance of the resultant display. Instead, it is desirable to coator print the encapsulated electrophoretic material combined with apolymeric binder in the form of a slurry on to a substrate and then todry and/or cure the coated or printed layer to form the final layer ofelectrophoretic medium. Although the coating or printing step can beperformed directly on the CFA, there may be problems with some CFA's inachieving proper packing of the capsules and/or sufficient adhesion ofthe capsules to the CFA.

In one aspect of this invention, the electrophoretic display material,or other solid electro-optic material, is coated, printed, or otherwisedeposited on to a release sheet for later transfer to a CFA or othersubstrate, for example a backplane (see also the aforementionedcopending application Ser. No. 10/249,957 and the correspondingInternational Application PCT/US03/16433). The display material isdesirably coated such that there is a thin adhesive layer between thedisplay material and the release sheet; a second adhesive layer may beprovided on the opposed side of the display material capsules. Toprotect the coated composite material during handling, it is desirableto apply a second release sheet over the second adhesive layer.

A preferred double release sheet (generally designated 300) of thepresent invention is shown in FIG. 3 of the accompanying drawings. Thissheet 300 comprises a central layer 302 of electro-optic material,specifically in FIG. 3 a layer comprising capsules 304 in a polymericbinder 306. The capsules 304 may be similar to those described abovewith reference to FIG. 1. The sheet 300 further comprises a firstadhesive layer 308, a first release sheet 310 covering the firstadhesive layer 308, a second adhesive layer 312 disposed on the opposedside of the layer 302 from the first adhesive layer 308, and a secondrelease sheet 314 covering the second adhesive layer 312.

The sheet 300 may be formed by first coating a the release sheet 310with a layer of adhesive which is then dried or cured to form the firstadhesive layer 308. Next, a mixture of the capsules 304 and binder 306is printed or otherwise deposited on the first adhesive layer 308, andthen the mixture is dried or cured to form a coherent layer 302.Finally, a layer of adhesive is deposited over the layer 302, dried orcured to form the second adhesive layer 312, and covered by the secondrelease sheet 314.

It will be apparent to those skilled in coating technology that thissequence of operations used to form the sheet 300 is well adapted forcontinuous production and that, by careful choice of materials andprocess conditions, it may be possible to carry out the entire sequenceof operations in a single pass through conventional roll-to-roll coatingapparatus.

To assemble a display using a double release film such as the film 300,one release sheet (typically that on to which the electro-optic materialwas coated) is peeled away, and the remaining layers of the doublerelease film are attached to a CFA or other front substrate using, forexample a thermal, radiation, or chemically based lamination process.Typically, the CFA or front substrate will include a conductive layerwhich will form the front electrode of the final display. The CFA andthe conductive layer may be in either order, but for reasons alreadydiscussed, it is preferred that the CFA lie between the conductive layerand the electro-optic layer. The front substrate may include additionallayers, such as a UV filter or a protective layer intended to protectthe CFA and/or the conductive layer from mechanical damage. Thereafter,the other release sheet is peeled away, thereby exposing the secondadhesive layer, which is used to attach the CFA/electro-optic materialcoating assembly to a backplane. Again, a thermal, radiation, orchemically based lamination process may be used. It will be appreciatedthat the order of the two laminations described is essentially arbitraryand could be reversed, although it practice it is almost always moreconvenient to laminate the double release film to the CFA or other frontsubstrate first, and thereafter to laminate the resulting frontsubassembly to the backplane.

As discussed above, in all CFA-based electro-optic displays, it isdesirable to make the distance from the color filter back surface to theoptically active layer as small as possible. This minimizes opticallosses and mitigates parallax problems. Thus, the lamination adhesive onthe front of the display and the capsule wall should both be as thin aspossible. It is conceivable that these layers may be reduced tothicknesses approaching 1 μm. Furthermore, it is desirable for thecapsules to deform during coating and drying such that they present aneffectively flat surface against the color filter array. This will helpminimize the chance that light entering through one color pixel leavesthrough an adjacent (differently colored) pixel after diffuselyreflecting form the optically active display material. Moreover, it isdesirable to match the index of refraction of all films used inconstructing the display stack.

Various display assembly techniques may be employed, including but notlimited to wet bond or hot melt lamination processes. In someapplications, it is preferable to use repositionable adhesives or airsoluble adhesives. In other applications, it is preferable to useradiation, thermal, or chemically curing adhesives. In each assemblytechnique, however, it is essential to align the CFA sub-pixels to theTFT array electronics on the backplane. In order to achieve highresolution alignment (aligned within better than, say, 10-20 μm), onecan use a standard optical alignment system that aligns fiducial markson the backplane to fiducial marks on the CFA. Once the fiducials onthese two substrates are aligned, they are brought together withoutintroducing misalignment and the lamination process takes place. Thelamination process may be based on 1. mechanical contact (i.e. pressuresensitive adhesives, either repositionable or otherwise), 2. thermaleffects (e.g. hot melt, wet bond, vacuum lamination, etc.), or 3.radiation-based methods (e.g. UV cure).

It will readily be apparent to those skilled in the technology ofelectro-optic displays that numerous changes and modifications may bemade in the process described for forming an electro-optic display froma double release film. For example, in each of the two laminationsdescribed, only one of the two components being laminated together needcarry an adhesive layer and which component carries the adhesive layeris essentially a matter of process engineering. Thus, in certain cases,it may be convenient to omit one or both of the adhesive layers 308 and312 from the sheet 300 and instead to place a similar adhesive layer onthe CFA, backplane or other substrate being used in the lamination.Also, in some cases it may be possible to omit one release sheet wherethis would not result in an adhesive layer being exposed tocontamination for an extended period. For example, if a double releasesheet such as the sheet 300 were formed on a continuous production lineand laminated to a color filter array or other substrate a short timeafter the second adhesive layer 312 was formed, application of thesecond release sheet 314 could be omitted and the second adhesive layerused to laminate the sheet 300 to the color filter array.

It will also be apparent to those skilled in the technology ofelectro-optic displays that the double release sheet of the presentinvention may be regarded as a modified form of the front plane laminatedescribed in the aforementioned copending application Ser. No.10/249,957 and the corresponding International ApplicationPCT/US03/16433. This front plane laminate comprises, in order, alight-transmissive electrically-conductive layer (typically carried on apolymeric film substrate), a layer of a solid electro-optic medium inelectrical contact with the electrically-conductive layer, an adhesivelayer, and a release sheet. The double release sheet of the presentinvention is essentially a modification of such a front plane laminatederived by replacing the electrically-conductive layer thereof with asecond release sheet, and optionally an associated second adhesivelayer. Accordingly, the double release sheet of the present inventionmay include any of the optional features of the front plane laminatedescribed in the aforementioned copending application Ser. No.10/249,957 and the corresponding International ApplicationPCT/US03/16433. For example, the double release sheet may have any oneor more of (a) a conductive layer on one of both of its release sheetsto permit testing of the electro-optic medium (see FIGS. 2-7 and theassociated description of application Ser. No. 10/249,957); (b) aconductor extending through the layer of electro-optic medium (see FIGS.9 and 10 and the associated description of application Ser. No.10/249,957); (c) an aperture extending through the layer ofelectro-optic medium, which aperture may later be filled with aconductive material to provide a conductor as mentioned in (b) (seeFIGS. 8 and 18 and the associated description of application Ser. No.10/249,957); (d) an edge seal (see FIGS. 11-17, 19 and 20 and theassociated description of application Ser. No. 10/249,957); and (e) tabsin which the release sheet extends beyond the layer of electro-opticmaterial to facilitate removal of the release sheet (cf. FIGS. 21 and 22and the associated description of application Ser. No. 10/249,957).

Edge seals around the perimeter of a display are desirable for displaylifetime and ruggedness reasons. The edge seal material may include UV,thermal, or chemically cured adhesives that are compatible with theencapsulated electrophoretic display material. The adhesive may bedeposited around the edge of the display using printing processes,automated pipette dispensing techniques, or other similar techniquesknown by those skilled in the art. In a traditional liquid crystalmanufacturing process, this seal material is cured before the liquidcrystal is filled into the cell gap. This is unacceptable for anencapsulated display because such an edge seal locks voids (e.g. airgaps) into the laminated display. There are several routes to eliminatethis problem. First, after lamination is complete one may fill the edgeseal material into the thin gap around the edge of the display. Second,one may only position edge seal material around some fraction of thedisplay edges, perform the lamination, then fill in the remaining edgeseal material. These and other edge sealing techniques are described inthe aforementioned copending application Ser. No. 10/249,957. However,the double release film of the present invention allows an additionalmethod for providing an edge seal, namely extending the adhesive layerslying on opposed sides of the electro-optic layer beyond the edge of theelectro-optic layer, but not beyond the edge of the display. FIG. 4illustrates an edge seal formed in this manner. FIG. 4 illustrates adisplay (generally designated 400) formed from a double release film,similar to as the film 300 shown in FIG. 3, by the double laminationprocess already described. The final display 400 comprises theelectro-optic layer 302, a backplane 316 and a CFA 318. The two adhesivelayers 308′ and 310′ extend beyond the edge of the electro-optic layer,but not beyond the edge of the display, so that after the twolaminations, the two adhesive layers become bonded together to form anedge seal, designated 320. Edge sealing is often essential in providinga rugged electro-optic display which can withstand wide variations inenvironmental conditions and it is much simpler to use the existingadhesive layers in this manner to form the required seal, rather thanintroducing a separate seal material around the outer edge of thedisplay.

Part F—Production of Color Filter Arrays by Photographic Imaging

In another aspect this invention provides a process for preparing acolor filter for use in an electro-optic display. The process comprisesimaging a color filter pattern on to a photosensitive film, processingthe film (if necessary) to reveal the image, then depositing aconductive layer on to the photosensitive film to act as an electrode.The term “photosensitive film” as used herein is not restricted to filmswhich rely upon silver halide chemistry, nor is it restricted to filmswhich are sensitive to visible wavelengths, but includes films sensitiveto electromagnetic radiation outside the visible range. If thephotosensitive film used is a conventional silver halide film comprisinga silver halide emulsion of a substrate, the conductive layer willtypically be deposited on the emulsion side of the film. The result is aflexible color filter that can be used as the top plane of anelectro-optic display.

The need for preparing CFA's on flexible substrates, and the problemsencountered in attempting to modify the conventional processesoriginally developed for forming CFA's on glass for use with liquidcrystal displays have already been discussed above. As already noted,such processes are not well suited to flexible substrates, and the costof production is also high, comparable to the cost of an amorphoussilicon TFT backplane on quartz.

In one embodiment of the invention the image is transferred by contactprinting an existing color filter array on glass on to the photographicfilm. The film is placed on a flat, non-reflective surface with theemulsion side up. The color filter is then placed dye side down on topof the film. The film is exposed with a light source through the colorfilter. This method requires an existing color filter to act as amaster.

Another embodiment of the invention for producing a filter uses a camerato image a pattern onto the film. The pattern could be an existing colorfilter, in which case it would be backlit. In this case, the camerawould be set up to reproduce the image 1:1 on the film. Alternatively,the master could be a large (poster-sized) reflective object, e.g. ahigh-quality print on paper. In this case, the master would befront-lit, and the camera would be set up to reduce the image to theappropriate size on the film. This method has the advantage of allowingthe rapid construction of a CFA from a macroscopic pattern, making itpossible to change such parameters as filter density, size of blackmask, etc.

Another embodiment of the invention uses a linear array of emissiveelements translated across the film in a direction perpendicular to thelong axis of the linear array to produce a pattern of stripes on thefilm. If a black mask is required in this embodiment, the emissiveelements could be toggled on and off at the appropriate points togenerate dark regions on the film.

In any of the above embodiments, the overall color cast of the filtercan be reduced (or enhanced) by applying an appropriate color filter tothe illumination source, or by placing filters over the lens of thecamera (when used). The density of the filter can be adjusted byincreasing or decreasing the film exposure. If the film does notaccurately reproduce one or more of the colors in the filter (which islikely), then it will be necessary to adjust the colors in the master.For any film, there will be an inverse transform governed by thespectral response function of the emulsion that will determine theproper colors to use in the master to get the desired colors in thefinal display.

To complete the construction of a CFA, after one of the processesdescribed above, the film may be processed using the appropriatechemical process. Then, a thin film of indium tin oxide (no) or otherconductive material can be evaporated on to the emulsion side of thefilm. Alternately, a transparent conductive polymer, e.g. Baytron(Registered Trade Mark), could be coated on to this surface.

The present invention permits the inexpensive production of CFA's onflexible substrates. Furthermore, using the reduction techniquedescribed above, CFA's of arbitrary geometry and color can be producedusing very simple and easily available tools, allowing for rapidprototyping of filter modifications.

Part G—Miscellaneous Processes for Production of Color Filter Arrays

This invention relates to various methods for integratingelectrophoretic display materials into a CFA based electrophoreticdisplay. The manufacturing strategies of this invention are markedlydifferent from those used in the manufacture of liquid crystal displays.

In a first aspect of the invention, an encapsulated electrophoreticdisplay material is coated directly onto the CFA and the coated CFA issubsequently laminated to a backplane containing pixel electrodes.

The deposition of the encapsulated electrophoretic display material(i.e., a plurality of capsules) may be effected by slot die, meniscus,curtain, or other coating methods directly on to the color filter array.The color filter array may be comprised of a glass or other opticallyclear substrate (including polymeric substrates and thin, “flexible”glass), a plurality of red, green, and blue color stripes, and atransparent conductor. In some embodiments, the color filter arrayincludes a “black mask”, which is a grid of opaque lines that aredesigned to conceal select undesirable locations on the assembledelectronic display. In still other embodiments, the color filter arrayis comprised of a plurality of red, green, and blue pixels, instead ofstripes. The black mask may be situated above or below the color stripesor pixels. In another embodiment, the color filter may include stripesor pixels that are optically clear instead of colored.

It is important to ensure that the encapsulated electrophoretic displaymaterial is in intimate contact with the CFA in order to ensure adequateoptical coupling between the display material and the color filter. At aminimum, it is required that the distance between the color filtersurface and the front surface of the optically active material in theencapsulated electrophoretic display material be substantially smallerthan the smallest dimensions of the pixels of the display. For example,for pixels that are 100 μm wide, one should ensure that the distancefrom the surface of the color filter stripe to the front surface of theoptically active material be smaller than, say 10 μm. This gap may befilled with lamination adhesive, a polymeric binder, capsule wallmaterial, thin films above the tinted regions of the color filter, andsurface treatments applied to the color filter.

A further aspect of the invention relates to methods for generatingregular surface energy patterns on the CFA to influence the packingdensity of the capsules on the CFA. According to this aspect of theinvention, a surface treatment may be applied to a CFA in order toinfluence the packing of capsules that are subsequently coated directlyonto the surface treated CFA. Generally, it is desirable to design thecolor filter such that there exists a regular surface energy patternover the surface of the color filter in contact with the encapsulatedelectrophoretic display material. This helps to ensure that the capsulespack in a way that improves the optical performance of the display.

Alternatively or in addition to such generation of regular surfaceenergy patterns, the surface may be prepared in such a way as touniformly increase or decrease wettability of this surface. This may beeffected by applying an adhesion promoter or suitable surface chemistry,such as 1-propanamine, 3-(trimethoxysilyl) (more systematically named3-aminopropyl-trimethoxysilane), 3-aminopropyldimethyl-ethoxysilane,hexamethyldisilizane or other such materials. Similar surface treatmentsmay also be applied to the surface of the backplane (or other surfaceswhere variation of wettability is desired, as described below) in orderto enhance display properties.

In a particular embodiment of this invention, the CFA may be constructedas follows: 1. pattern red, green, and blue stripes on a substrate, 2.pattern a black mask above the color stripes (the black mask isconventionally situated below the color stripes), and 3. treat the blackmask surface such that it becomes non-wetting and the color stripeswetting to the encapsulated electrophoretic display material.Representative surface treatments include octadecatrichlorosilane, aspectrum of other silane and thiol based chemicals, variouspolytetrafluoroethylene agents, and other materials known to thoseskilled in the art. The black mask may be made receptive to these agentsby including a surface treatment receptive film on the topmost surfaceof the black mask. For example, a thin gold film may be deposited as thetopmost layer of the black mask, and alkanethiols may be deposited ontothe gold. The thiol material may be engineered to have a markedlydifferent surface energy than the color stripes, thereby achieving thedesired effect.

A further aspect of the invention relates to methods for using patternedsurface treatments to influence the packing density of the coatedcapsules on a continuous electrode. In this aspect, a patterned surfacetreatment is deposited onto a continuous electrode to influence thepacking of subsequently deposited capsules. A patterned surfacetreatment may be applied onto the continuous electrode on top of thecolor filter by printing techniques including microcontact, offset,intaglio, flexo-gravure, or other printing techniques known to thoseskilled in the art. This is an ideal application for printing, asperfect yield on the local scale is not required; local defects are notcatastrophic, since proper packing of the capsules on the long rangewill tend to organize the capsules correctly despite local defects.

In another aspect, this invention relates to the use of monodispersecapsules (capsules that are substantially uniform in size, for examplein which at least about 95 per cent of the capsules have diameters whichdo not differ by more than about 20 per cent, and preferably by not morethan about 10 per cent, from the average diameter) in electrophoreticdisplays. The use of such monodisperse capsules is especially desirablein conjunction with the surface treatment aspects of this inventionalready discussed, since with polydisperse capsules, regularly patternedsurface treatments become far less effective at reordering the depositedfilm into a regular pattern, whether these surface treatments aredeposited onto a color filter black mask or onto a continuous electrode.

These aspects of the present invention all serve to improve the opticalperformance of a encapsulated electrophoretic display by influencing thepacking of capsules coated down onto a front electrode. The surfacetreatments used to accomplish this task are either built into the colorfilter black mask, or are printed directly onto the continuous electrodein contact with the display material.

Another aspect of the present invention relates to novel surfacetreatments that enable filling a cell gap in between a CFA and abackplane with encapsulated electrophoretic display material. The novelsurface treatment technique of the present invention enables the use ofmore traditional liquid display manufacturing techniques. In this aspectof the invention, a surface agent that promotes capsule wetting isapplied to the CFA, and a surface agent that promotes capsule dewettingis applied to the backplane. Just as in traditional liquid crystalmanufacture, the color filter array and the backplane are assembled withprecision spacers in between them, and the edges are sealed with epoxyor other edge-sealing agent. The encapsulated electrophoretic displaymaterial in then drawn into the space between the color filter array andthe backplane using pump, vacuum, or other similar technique known tothose skilled in the art. However, because of the presence of thesurface treatments on the CFA and the backplane, the capsulespreferentially pack tightly against the front surface of the display(the color filter surface), and tend to dewet and move away from thebackplane surface. This invention ensures that the encapsulatedelectrophoretic display material packs against the front viewingsurface, which provides for markedly improved optical performancecompared to techniques in which the material packs against thebackplane, or packs with no preference for either the CFA or thebackplane surface. Surface forces are used to draw capsules to the frontelectrode, but other forces including gravitational, electrophoretic,and magnetic may be employed to accomplish this task.

This aspect of the present invention improves the optical performance ofthe display by influencing the packing of capsules injected into a cellgap formed between a front electrode and a backplane. It enables the useof traditional display filling technologies with encapsulatedelectrophoretic displays.

The electrophoretic media used in various aspects of the presentinvention may be of any of the types described in the aforementioned EInk and MIT patents and applications, to which the reader is referredfor further information.

Those skilled in the part of electro-optic displays will appreciate thatnumerous changes, improvements and modifications can be made in thepreferred embodiments of the invention already described withoutdeparting from the scope of the invention. Accordingly, the whole of theforegoing description is intended to be construed in an illustrative andnot in a limitative sense.

1. A process for forming an electrophoretic display, the process comprising: providing a color filter array; providing an electrophoretic medium comprising a plurality of capsules, each capsule comprising a suspending fluid, a plurality of electrically charged particles suspended in the suspending fluid and capable of moving therethrough on application of an electric field to the suspending fluid, and a capsule wall surrounding the suspending fluid and the electrically charged particles; depositing the electrophoretic medium on the color filter array to form a coated color filter array; and thereafter laminating the coated color filter array to a backplane comprising at least one pixel electrode.
 2. A process according to claim 1 wherein the surface of the color filter array is surface treated prior to the deposition to produce regions of varying surface energy on the surface. 